1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, more particularly to a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer, with the transparency or color thereof being reversibly changeable depending upon the temperature thereof, which is capable of repeatedly recording information therein and erasing recording information therefrom by utilizing the reversibly changeable transparency or color of the reversible thermosensitive recording layer. The present invention also relates to a method of forming images in the above-mentioned reversible thermosensitive recording medium and erasing the same therefrom by applying a laser beam thereto.
2. Discussion of Background
Recently, reversible thermosensitive recording media, which are capable of temporarily forming images therein and also capable of deleting formed images therefrom when such formed images becomes unnecessary, have attracted attention.
Japanese Laid-Open Patent Applications 54-119377 and 55-154198 disclose representative examples of such a reversible thermosensitive recording medium, which comprises an organic low-molecular weight material such as a higher fatty acid, which is dispersed in a matrix resin such as a vinyl chloride-vinyl acetate copolymer.
However, such a reversible thermosensitive recording medium has the shortcoming that the surface of the reversible thermosensitive recording medium takes scratches when a thermal head is employed as a heating element, and therefore, it becomes difficult to form uniform images in the recording medium during repeated image formation and erasure. This is because such a heating element is rubbed against the surface of the recording medium with the application of heat thereto.
In order to decrease the scratches on the surface of the recording medium when the thermal head is used as the heating element, the inventors of the present invention have proposed the provision of a protective layer on the surface of the recording medium, as disclosed in Japanese Laid-Open Patent Applications 63-221087, 63-317385 and 2-566. However, the provision of the protective layer is not enough to protect the surface of the recording medium from the scratches when the image formation and erasure are repeated many times.
The other shortcoming of the conventional reversible thermosensitive recording medium in which the organic low-molecular-weight material is dispersed in the matrix resin is that the organic low-molecular-weight material tends to aggregate, and the milky whiteness degree of the reversible thermosensitive recording layer is therefore gradually decreased as the image formation and image erasure are repeatedly carried out by simultaneously applying heat and pressure to the recording medium, for example, using the thermal head.
To prevent such deterioration of the recording medium, there is known a method of heating a reversible thermosensitive recording layer of the recording medium not in contact with a heating element. According to the above-mentioned non-contact heating method, the reversible thermosensitive recording layer is softened by the application of heat thereto, but not impaired because no pressure is applied thereto, thereby preventing the deterioration of the reversible thermosensitive recording medium. For instance, the recording is carried out in the reversible thermosensitive recording medium by use of a laser beam as disclosed in Japanese Laid-Open Patent Application 57-82088. In this case, carbon black and a resin such as ethylcellulose are contained in the reversible thermosensitive recording layer or a layer adjacent to the reversible thermosensitive recording layer. This method enables the recording to be carried out by the non-contact heating system. However, the images formed in the reversible thermosensitive recording medium become grayish as a whole and the image contrast is considerably poor not only when the carbon black is added to the reversible thermosensitive recording layer, but also when it is added to the layer adjacent to the reversible thermosensitive recording layer.
In addition, as disclosed in Japanese Laid-Open Patent Application 64-14077, it is proposed to add a dye to the reversible thermosensitive layer or provide a dye-containing layer or metallic layer capable of absorbing near infrared rays in immediate proximity to the reversible thermosensitive layer.
When the dye is contained in the reversible thermosensitive recording layer, the contrast of the obtained images is not sufficient for practical use although it becomes better as compared with the case where the carbon black is employed.
When the carbon black or dye is contained in the reversible thermosensitive layer or the layer adjacent thereto, a thermoplastic resin is generally used in combination with the carbon black or dye in the layer. Therefore, when the laser beam is applied to the recording medium for recording operation, a very tiny area is instantaneously heated to high temperature and the thermoplastic resin is softened, with the result that the layer will be deformed as a whole.
When the near-infrared-rays-absorbing layer made from metals such as Se, Ge and Cr is provided adjacent to the reversible thermosensitive layer, the problem of thermal deformation does not occur, but the image contrast is decreased because the metallic luster of the above-mentioned metals is relatively low. In addition, the above-mentioned metals have toxicity, so that the recording medium cannot be discarded without any treatment when it becomes unnecessary.